1. Field of the Invention
The present invention relates to thermal mass transfer compositions and articles for use in producing signage articles. In particular the present invention relates to the use of thermal mass transfer compositions which employ a binder which does not significantly absorb ultraviolet radiation, thereby avoiding substantial heat absorption and degradation of the binder, and including durable, stable pigments which do not exhibit significant color change or loss of brightness.
2. Related Art
Thermal mass transfer processes use a donor sheet (commonly denoted a "ribbon" or "foil") and a receptor sheet or substrate. The thermal mass transfer donor sheet normally comprises a carrier layer and a colorant layer with at least a thermally transferable colorant (a dye or preferably a pigment) in a heat softenable binder. The colorant layer typically consists of a pigment dispersed in a binder (the binder transferring with the pigment during thermal transfer). Thermal mass transfer sheets are used with the donor surface (colorant layer) in intimate contact with a receptor material, and the donor sheet is heated in an imagewise manner (e.g., by thermal printheads, irradiation as by a laser or high intensity radiation transmitted through a mask or stencil) to transfer the image forming material. In the thermal mass transfer system, the colorant layer is softened by the imagewise heating (and sometimes a receptor layer on the receptor sheet is contemporaneously softened), and the softened area is transferred to the receptor sheet. The ultimate use of the substrate having the transferred image thereon frequently dictates the durability requirements of the image.
Thermal mass transfer is useful for preparing outdoor durable signage articles, such as automobile registration tags which are adhered to license plates. See, for example, Patent Cooperation Treaty applications WO 94/19710 (claiming priority from U.S. Ser. Nos. 08/017,573 and 08/033,627) and WO 94/19769 (claiming priority from U.S. Ser. Nos. 08/017,573 and 08/033,625), both published Sep. 1, 1994. For the articles described therein, thermal mass transferred indicia was printed onto a specially formulated polyurethane "multifunction" layer, an attribute of which was to eliminate the need for a cover layer for resin-based binder printed indicia. Many commonly used binders employ large amounts of UV stabilizers and UV absorbers, which tend to produce heat in the binders and cause their early degradation, thereby decreasing brightness and/or increasing color change.
There is a need in the signage art, particularly those meant for outdoor usage, to be able to apply color images to many different substrates without loss of brightness or color change, and without having to use complex processes.